The present invention relates to an electrical filter wherein acoustic waves run close to a surface in a substrate thereof, and wherein at least one input transducer and at least one output transducer are provided in association with the substrate. An arrangement comprising two identically weighted transducers is specified in FIG. 7 in IEEE Transactions on Sonics and Ultrasonics, Mar. 1975, pages 105 through 114, incorporated herein.
Electrical filters with acoustic waves running in a substrate close to the surface and having an input transducer and an output transducer are known. What is meant by "running close to the surface" are not only surface waves (Raleigh and Bleustein waves) in the narrower sense, but also Love waves, SSBW, SBAW waves, and the like. For such acoustic waves, it is standard to employ fingers or electrode structures which are arranged on the surface of the substrate, and executed in accordance with the intended use. Such finger structures, namely interdigital transducer structures, reflector (strip) structures and further structures such as multi-strip coupler and the like, correspond in arrangement, dimensioning, and design to the rules prescribed by the given transfer function, resonator function, and the like.
The employment of finger overlap weighting is known for the use of a prescribed transfer function, this finger overlap weighting being provided such that the fingers of the comb structures of an interdigital transducer structure which engage into one another have a mutual overlap length of a greater or lesser size. It is standard to fill out non-overlapping finger lengths by dummy finger lengths in order to achieve uniformity of the surface occupation of the substrate surface with electrode fingers (see "Siemens Forschungs-Und Entwicklungs-Berichte, Volume 6 (1977), pages 132 through 136 (FIG. 5), incorporated herein.
In the following, electric filters with acoustic waves shall be referred to in summary as surface wave filters, even though this term is also understood in a narrower sense.
Surface wave filters, namely both transversal filters as well as resonator filters or resonators, are usually provided with such transducers as an input transducer and an output transducer. The first transducer has a finger overlap weighting and the second transducer is an unweighted transducer, i.e. its finger overlaps are maximum and identical for all fingers. Such a design, however, has the disadvantage of high weighting losses and higher insertion attenuation. This is particularly disturbing especially in resonator filters. Transversal modes which occur are also disturbing therein.
In an entirely different context than of the invention, Matthews, Surface Wave Filters, Design, Construction and Use, Verlag Wiley 1977, FIG. 3.13 on page 134, incorporated herein, discloses a filter with a finger-overlap-weighted input transducer and a finger-overlap-weighted output transducer. However, this filter necessarily forms a multi-strip coupler and thus does not comprise an in-line structure. The overlap weighting of both transducers, moreover, is at least practically identical.
A filter which likewise comprises a finger-overlap-weighted input transducer and a finger-overlap-weighted output transducer is also known from the above book, pages 131/132, incorporated herein. The overlap weightings of these two transducers are again at least essentially identical. In this filter, the dimensioning of the finger-overlap weightings of the input transducer and the output transducer is extremely difficult, since it is only accessible to an indirect mathematical acquisition, and one is relatively largely dependent on trial and error in this dimensioning. This disadvantage is cited in the above cited publication, in addition to further disadvantages.